The invention relates to an acoustic detection device for receiving and processing acoustic signals, which comprises:
k acoustic detectors for the detection and conversion of acoustic signals into electrical signals s(k); PA1 beamforming means and a frequency analysis unit which uses the signals s(k) to generate signals s(.phi..sub.i, f.sub.j) which represent acoustic signals with n frequencies f.sub.j (j=1 . . . n) received from m beam reception directions .phi..sub.i (i=1 . . . m); PA1 a data processing unit suitable for processing the signals s(.phi..sub.i, f.sub.j).
The invention also relates to a data processing unit, a background normalisation unit, a direction estimator unit and an automatic gain control unit suited to be used in such an acoustic detection device.
A device of the type mentioned above is known from U.S. Pat. No. 4,207,624. It describes a sonar apparatus suitable for the detection of sonar signals. For this purpose the instrument is provided with various acoustic detectors embodied by hydrophones, a beamformer connected to them and a frequency analysis unit which is connected to the beamformer and operates according to with the FFT principle. The digitised signals obtained for each beam direction in a frequency domain by the frequency analysis unit are subsequently presented to the data processing unit. The data processing unit comprises for each beam direction and each frequency component a filter which filters, the signals supplied successively to the filter, with respect to time. Signals that are stationary in time, are transferred in an amplified way compared to non-stationary signals in time. This is achieved by multiplying each digitised signal by a complex number which is determined adaptively in a feedback loop. This results in an improved signal-to-noise ratio as non-stationary noise in time is attenuated.
A disadvantage of this filtering process is that background and interference signals stationary in time are not attenuated, while components of a target signal which are not stationary in time are attenuated.
Furthermore, the way an accurate estimation of a direction of a received target echo can be determined and a target can be tracked is not disclosed for the device mentioned.
It is generally known that the estimation of a direction, by means of interpolation, on the basis of signals received in different and adjacent beam directions, is many times more accurate than one beamwidth. The signal thus obtained for each beam direction can represent either a broad frequency range or a narrow frequency range. The former instance is called a "broadband" estimation and the latter a "narrowband" estimation of the direction.
A disadvantage inherent to broadband estimations is that when a high background level occurs at low frequencies and target signals occur at high frequencies, the target signal contributes relatively little to the sum signal. Particularly in the case of low frequencies, high background levels will occur due to the better propagation characteristics of sonar signals at low frequencies. However, a well-known solution to this problem is to apply a so-called fixed pre-whitening filter, which "whitens" the frequency spectrum. This does not work optimally for background levels whose frequency dependence varies across the time. An additional disadvantage of broadband estimations is the sensitivity to interference signals, resulting from the large bandwidth.
Narrowband estimations, however, only work optimally if the target signal has striking fixed frequency components which can be tracked. If these are lacking or change frequency, narrowband estimations are no longer optimal. Besides, interference signals present in the frequency band used, can affect the measurement, especially in case of broadband interference.